1. Field of the Invention
The present invention relates to an optical head, and more particularly to an optical head capable of recording/reproducing data to and from a variety of optical disks each having a different available wavelength or a base material of different thicknesses.
2. Description of the Related Art
A standard optical head using a laser diode (LD) is described with reference to FIG. 20. A light beam 203 output from an LD 201 is collimated by a collimate lens 202. The collimated light beam 203, which is a P-polarized light beam, is transmitted through a polarized beam splitter 204 (hereinafter, referred to as "PBS"), and then is incident onto a quarter-wave plate 205. The light beam 203 output from the quarter-wave plate 205 is, via a reflecting mirror 206, incident onto an objective lens 207. The light beam 203 is converged into an imaging point p by the objective lens 207, thus forming a beam spot 209 on the recording face of an optical disk 208. Then, a light beam 210, i.e., the light beam 203 reflected by the optical disk 208, is incident onto the objective lens 207 again. After passing though the reflecting mirror 206 and the quarter-wave plate 205 in this order, the light beam 210 is incident onto the PBS 204. At this stage, the light beam 210 has changed into an S-polarized beam by the quarter-wave plate 205 and is reflected by the PBS 204 without transmitting therethrough. The reflected light beam 210 passes through a detection lens 211 and a cylindrical lens 212, and is incident onto a photodetector (hereinafter referred to as "PD") 213. The PD 213 detects a reproducing signal based on the incident light beam 210. At the same time, the PD 213 detects a focus control signal and a tracking control by using known methods. For example, the focus control signal is obtained by an astigmatism method, and the tracking control signal is obtained by a push-pull method.
The objective lens 207 used in such an optical head is designed in consideration of the thickness of the base material of the optical disk 208 and the available wavelength thereof. This is because a wavefront aberration arises if the base material thickness or available wavelength of the optical disk 208 is different from the base material thickness or available wavelength, a factor which is considered in designing the objective lens 207, a factor which can result in failure of the recording/reproducing operation.
Conventionally, the base materials of a compact disk (hereinafter, referred to as "CD"), a video disk, an optical disk used for a magneto-optic disk drive, etc., all have been 1.2 mm in thickness. In addition, the wavelength of a light beam used for the recording/reproducing operation of such optical disks (hereinafter, referred to as "the available wavelength") has been 780 nm to 830 nm. Accordingly, as for the conventional optical disk, optical disks of various types can be recorded and reproduced by using the same optical head.
In recent years, in order to realize an optical disk with a higher recording density, attempts have been made to increase the numerical aperture of the objective lens, to use a light beam of a shorter wavelength, and the like. However, it is difficult to perform the recording/reproducing operation of the high density recording optical disk using an optical head identical with that used for the conventional optical disk.
First, by increasing the numerical aperture of the objective lens, the wider frequency band allowing the light beam to be reproduced is realized due to improvement in the optical resolution. However, in such a case, if the recording face of the optical disk 208 is inclined with respect to the plane perpendicular to the optical axis of the objective lens 207, the coma of the light spot 209 increases. Thus, the increase of the numerical aperture of the objective lens does not actually improve the image forming efficiency.
In order to increase the numerical aperture of the objective lens without increasing the coma aberration, an optical disk having a thinner base material may be used. FIG. 21 is a graph showing the correlation between the thickness of the base material of the optical disk 208 and the numerical aperture of the objective lens 207. The curve in FIG. 21 is constituted by the points at which the peak value of the light intensity distribution of the light spot 209 is a predetermined value when the recording face of the optical disk 208 is inclined by 0.2.degree. from the plane perpendicular to the optical axis of the objective lens 207.
As seen from FIG. 21, the decline of the peak value due to the inclination of the optical disk 208 in the case where the beam spot 209 is formed by the objective lens 207 with a numerical aperture of 0.5 on the optical disk whose base material is 1.2 mm thick is substantially equal to that in the case where it is formed by the objective lens 207 with a numerical aperture of 0.62 on the optical disk whose base material is 0.6 mm thick. Accordingly, even if the numerical aperture of the objective lens 207 increases, by making the base material of the optical disk thinner, the coma resulting from the inclination of the optical disk can be reduced so as to be in the same degree as the coma of the conventional disk. However, when making the base material of the optical disk thinner, the wavefront aberration makes it impossible to use the same optical head to perform the recording/reproducing operations for the optical disks having the base material of 1.2 mm in thickness and those having thinner base material. That is, interchangeability between optical heads is lost. As a result, in order to perform the recording/reproducing operations for optical disks having a base material of 1.2 mm in thickness and those having a thinner base material by one optical data recording/reproducing apparatus, the optical data recording/reproducing apparatus is required to have two different optical heads, one for the optical disk of the former thickness and the other for that of the latter thickness.
Second, in the case of applying a light beam with a shorter wavelength for the recording/reproducing operation, an improved optical resolution allows the widening of the frequency band assuring the recording or reproducing operation. However, for the purpose of reproducing data from a conventional optical disk whose available wavelength is 780 nm, if a light beam of a shorter wavelength, e.g., 635 nm, is used, a reproduction signal or control signals of a sufficient level cannot be obtained due to the difference in the reflectance or absorption rate of the recording face of the optical disk. This problem is prominent, for example, when using a light beam of a short wavelength for a CD-R standardized as a writable CD.
FIG. 22 is a graph showing exemplary data representing how the reflectance of the CD-R depends on the wavelength of the light beam. The CD-R is defined as having a reflectance of 65% or more with respect to a light beam having a wavelength of 775 nm to 820 nm. However, the reflectance is extremely lowered as for a light beam having a wavelength out of this range. In some types of CD-R, the reflectance becomes as small as 5% with respect to a light beam having a wavelength of about 635 nm. Furthermore, the reproduction power of the CD-R is defined to be 0.7 mW or less. As a result, when trying to reproduce data from the CD-R having the wavelength dependency of the reflectance as shown in FIG. 22 by an optical head with an LD generating a light beam of a wavelength of 635 nm, even assuming that the reproduction power is the upper limit value of 0.7 mW and that the efficiency of a reproduction optical system is 100%, only a power of 35 .mu.W is obtainable in the reproduction-detection system. Thus, in order to perform the recording/reproducing operation of the CD-R whose available wavelength is 780 nm by using the light beam of a wavelength of 635 nm, a signal reproducing system which has an extremely high S/N ratio is required and thus is expensive.
However, in consideration of the fabrication cost, an optical head of a widely marketed standard model is desired to have a reproduction system with an efficiency of 50% or less. Hence, it is difficult for the optical head of a standard model to assure a satisfactory reproduction S/N ratio, when using the light beam of a wavelength of 635 nm.
For these reasons, it has been very difficult to use a single optical head to perform the recording/reproducing operation for both the high-density optical disk available for a light beam of a wavelength of 635 nm and the conventional optical disk available for a light beam of a wavelength of 780 nm. Accordingly, in order to perform the recording/reproducing operation for both the high-density optical disk and the conventional optical disk by a single optical data recording/reproducing apparatus, the apparatus has been required to separately provide the optical head using a light beam having a wavelength of 635 nm and the optical head designed for conventional optical disks. In addition, the apparatus has needed to provide optical systems respectively for both optical heads, for appropriately converging a light beam generated from each of the LDs onto the optical disk. This bulky configuration has necessitated high fabrication costs, as well as hindering the apparatus from being developed into a smallersize.